As is well known, any material will emit electromagnetic radiation, whose intensity as a function of wavelength depends on its temperature. This intensity is described by Planck's law.
Methods for measuring the electromagnetic radiation emitted by the material and for deducing its temperature have been developed for a long time. These methods may be qualified as “passive” measurement methods, since they measure only the temperature of the material, from which the predetermined physical properties are deduced.
These methods have the advantage of being non-intrusive, since they do not require physical contact between a measuring device and the material.
On the other hand, there are other proposed temperature measuring methods which do require contact between a measuring device and the material. These include, for example, the use of thermocouples. This type of measurement has other drawbacks. In particular, the measurement cannot be performed without advance preparation of the block of material.
Improved methods of the aforementioned “contactless” type have also been proposed in the Prior Art, including a method of measurement using so-called “active” pyrometry. A method of this type consists of heating the block of material using a heat source, for example a laser, and of measuring the radiation from the body that results from the heating in order to deduce its temperature. The radiation is analyzed using an electromagnetic radiation collecting device, a sensor that converts the electromagnetic energy into electrical signals and an electronic signal processing system. Analyzing the temperature and its time evolution makes it possible to deduce certain physical properties of the material, for example its thermal conductivity, its absorptivity, or in the case of a multilayer material sample, the thickness of the layers and the thermal resistance between two layers.
Methods of this type for analyzing the physical properties of materials by measuring the temperature using active pyrometry have been known for many years.
They include, to give a non-exhaustive list of examples, the following methods:
French patent application FR 2 593 917 A1 (UNIVERSITY OF REIMS-CHAMPAGHE-ARDENNE) describes a method for measuring the absorptivity, the diffusivity and the thermal resistance between two layers of material. The material sample is composed of a thin layer, which is transparent to the wavelength of the electromagnetic radiation, disposed on a thick substrate. The substrate is heated using a laser beam that is amplitude-modulated at a high frequency and a low frequency. The thermal emission from the substrate is then measured. Analyzing the phase difference between the thermal emission and the laser signal makes it possible to determine the physical properties of the transparent thin layer.
French patent application FR 2 647 547 A1 (UNIVERSITY OF REIMS-CHAMPAGHE-ARDENNE) describes a method for measuring the thermal contact resistance between two layers that are opaque to laser radiation. The surface of the material sample is heated using a modulated laser beam, and analyzing the modulated component of the surface temperature makes it possible to determine the value of the thermal contact between the two layers. However, this method requires that a prior calibration be performed on a set of representative test samples of the material to be analyzed.
French patent application FR 2 663 745 A1 (UNIVERSITY OF REIMS-CHAMPAGHE-ARDENNE) describes a method similar to that of the preceding patent application. However, the modulation of the laser beam is obtained using a pseudo-random binary signal.
The above-mentioned methods use modulated laser beams. Other methods are described in the literature as applying a pulsed heating signal, i.e., using for example a laser that emits short (a few nanoseconds), high-intensity pulses. These features offer several advantages. First, the resulting temperature increase is higher, thus making it possible to also obtain a higher electromagnetic radiation of the surface, and hence a better precision in the measurement, by increasing the signal-to-noise ratio.
Finally, another method of active pyrometry in the Prior Art proposes using a laser beam that is modulated and pulsed. Such a method was described, for example, by T. Loarer in the doctoral thesis entitled: Mesure de □odule□ture de surface par effet photothermique □odule ou impulsionnel [“Surface temperature measurement by means of a modulated or pulsed photothermal effect”], Ecole Centrale Paris, (1989).
This method was the subject of an Israeli patent application, filed under the number IL 1996118611, which corresponds to U.S. Pat. No. 5,957,581 (Katzir et al.).
These documents describe a method for measuring surface temperature by analyzing the temporal shape of the decrease in temperature after a laser shot, without taking into account the value of the maximum temperature. This method has the particular advantage of making it possible to dispense with the in situ calibration of the measuring device.
The above-mentioned methods of the Prior Art admittedly have advantages, but they do not entirely meet current needs in the fields of application for which the invention is intended. Moreover, while the last method described would, in theory, seem to be the most advantageous, it should noted that it is not easy to use for analyzing layers of material having thicknesses on the order of a few micrometers. In fact, when the material is heated using a pulsed laser, only a small thickness of material near the surface is actually heated, and the influence of the thickness of the thin superficial layer and of the thermal contact between this superficial layer and a substrate are not easy to establish. Finally, modulated laser beam heating, while admittedly making it possible to measure the physical properties of multilayer material samples, is a method that suffers from a low “signal-to-noise” ratio.